1. Field of the Invention
The present invention relates to a photothermographic material and a method of forming an image using the photothermographic material. More particularly, the invention relates to an improved photothermographic material, which exhibits stable photographic properties without unevenness in density, and an improved method of forming an image.
2. Description of the Related Art
In recent years, it has been strongly desired in the field of films for medical imaging to reduce the amount of used processing liquid waste in consideration of environmental protection and space saving. For this reason, technology regarding photothermographic materials as films for medical imaging and for photographic applications, which are capable of efficient exposure with a laser image setter or a laser imager and capable of forming a clear black-toned image with high resolution and high sharpness is desired. Such photothermographic materials can eliminate use of liquid processing chemicals and can provide users with a thermal development system which is simpler and does not contaminate the environment.
Although similar requirements also exist in the field of general image forming materials, an image for medical imaging requires a particularly high image quality excellent in sharpness and granularity because a delicate image representation is necessitated. Also an image of blue-black tone is preferred in consideration of easy diagnosis. Currently various hard copy systems utilizing pigments or dyes, such as ink jet printers and electrophotographic systems, are available as general image forming systems, but they are not satisfactory as output systems for medical images.
On the other hand, thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems”, written by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. A photothermographic material generally comprises a photosensitive layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt) and, if necessary, a toner for controlling the tone of a developed silver image are dispersed in a matrix of a binder. The photothermographic material, when heated at high temperature (for example, 80° C. or higher) after image exposure, forms a black-toned silver image by an oxidation/reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizer) and the reducing agent. The oxidation/reduction reaction is promoted by a catalytic effect of a latent image formed by exposure on silver halide. Thus, a black-toned silver image is formed in an exposed area. Such materials are described in U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924. Also, Fuji Medical Dry Laser Imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
In production of a photothermographic material using an organic silver salt, two methods are available: in one method, a solvent coating is adopted, and in the other method, an aqueous coating is adopted. It is known that in the aqueous coating method, a coating solution for an image forming layer containing an aqueous dispersion of polymer fine particles as a main binder is used. In the latter method, since no necessity arises for a process of solvent recovery or the like, a production facility is simple and the method is advantageous for mass production.
In the photothermographic material, all chemicals required for image forming are included in a coating film beforehand, and the chemicals remain as unreacted compounds or reaction products in the film after performing thermal development.
Therefore, when the photothermographic material is exposed to indoor light or the like after image formation or is exposed to high temperatures while being stored, the reductive reaction of silver ions occurs and results in fogging, which has been an intrinsic problem of photothermographic materials. This problem of image stability called “print-out” is specific to the photothermographic materials and improvements are still further required for the photothermographic materials.
JP-A No. 2001-33911 discloses that, for example, a polyhalogen compound which oxidatively decomposes unnecessary fogging silver generated in the processed photothermographic material over time is effective as means for improving image stability. JP-A Nos. 2002-156727 and 2002-318431 disclose a complex-forming agent which forms a complex with a developing agent and restrains undesirable reductive reaction during storage. However, these conventional techniques have limitations with respect to the improvement of print-out, especially in the presence of lighting, and therefore, technology for drastic improvement is desired.